help forwarding

[kanhu rauta]

Dear sir,

SORRY FOR MINE POOR ENGLISH.
this is mine configuration.
a----------------->b------------------------------------------->c  
           10.10.10.5(global ip)                        10.10.10.5
           port=7500                                    port=5500
           server s1                                    server s1 
>now server s1 is running on both the machine but on different port.
Objective
---------------------
>if 'a' sends a packet(ip=10.10.10.5 dport=7500) .then this packet is recive by 'b'
>Otherwise forward it to 'c'
---------------------------  
>i have made modification on the source code(also ip_forward enabled).
>for udp its no problem
>for tcp,one way forwarding is no problem.('a' sends a syn packet having destination ip=10.10.10.5 and dport=5500 to 'b'. now 'b' forwards it to 'c')

>now the ack packet 'c' sends to  'b'.but 'b' can't deliver it to 'a'.

>i have followed the same techineque for forwarding from 'b' to 'a'(as that of i do while forwarding 'b' to 'c') skiping the source validation .still it hangs sometimes not always.i gives the cases

CASE 1:
-------
>send a udp packet having(ip=10.10.10.5 port=5500) from 'a'.it reaches at 'c'
>now send a packet udp packet from 'c' to 'a'.it reaches
>now send a tcp packet from 'a'(ip=10.10.10.5 dport=5500).it reaches to 'c' also reply comes to 'a'.its working, its fine.

CASE 2:
-------
>if i first tries to send a tcp packet having(ip=10.10.10.5 port=5500)
then system 'b' hangs.

CASE 3:
---------
>if i first send a udp packet from 'c' to 'a'.system 'b' hangs.


i did not getting the problem.
With Regards
  kanhu